An optical resonator is an important element, which can be incorporated into many of the components used for optical communication systems such as lasers, filters, routers, switches, etc. Such resonator can be easily realized in integrated optical devices with linear waveguides to form a Planar Lightwave Circuit (PLC). One of the most common roles of the optical resonator is to serve as a wavelength dependent coupler between two (or more) waveguides (input/output (I/O) waveguides). This is schematically illustrated in FIGS. 1A and 1B, wherein an oval-like or circular resonator serves for coupling between two linear waveguides. The light couples from one (input) linear waveguide into the resonator waveguide and from the resonator to the other (output) linear waveguide. In specific wavelengths, known as the resonance wavelengths of the resonator structure, all the light is eventually transferred from the first linear waveguide to the second linear waveguide. The resonator is typically characterized by following parameters:                Free spectral, range (FSR);        Loss per revolution;        Coupling to the waveguides;        Q factor, which can be derived from the three parameters defined above.        
Two primary implementation types of micro-ring resonators in planar technology are known in the art:                1. Single layer implementation—both I/O waveguides and the resonator are located in the same layer (horizontal coupling as shown in FIGS. 1A and 1B). This is disclosed for example in WO 01/22139 and WO 00/72065;        2. Double layer implementation—the I/O waveguides and the resonator are located in different layers (vertical coupling—as shown in FIG. 1C). This approach is disclosed in the following publication: B. E. Little et al., “Vertically coupled glass microring resonator channel dropping filters”, IEEE Photonics Technology Letters vol. 11 no. 2, February 1999, p. 215-217). Here, ng is the refractive index of a substrate, ng, wg and hg are the refractive index, width and height of the input/output waveguides, nr, wr and hr are the refractive index, width and height, respectively, of the ring, and n0 is the refractive index of a cladding layer.        
Both implementations have advantages and disadvantages. Because of the vertical coupling mechanism, the double layer implementation may have better coupling and loss characteristics, as compared to those of the single-layer implementation, but requires complex and expensive process capabilities. The single layer implementation, although requiring simpler process, presents a design tradeoff between the resonator parameters (FSR, coupling, loss/rev). Characteristics such as large FSR, low loss and high coupling (more than 3%) are important for a micro-ring resonator, regardless of the specific function it fulfills. However, achieving these characteristics simultaneously is difficult since the demands on the resonator shape contradict.
To achieve low losses, the best resonator shape would be a perfect circular ring with high refractive index contrast to achieve tight mode profiles. However, these characteristics would result in poor coupling due to the tight mode profiles of the waveguides and the resonator, or would require a very small gap, which in turn requires very complex and expensive processes. In order to increase the coupling, the racetrack resonator, which is comprised of two straight waveguides connected with two half rings, was suggested (see FIG. 1A). Although this resonator shape improves the coupling between the I/O waveguide and the resonator, it also increases the losses per revolution (loss/rev) due to the mismatch between the straight waveguide and the circular waveguide modes.